Girls just wanna have fun. And girls just wanna get laid. Hey, there are no judgments in Marielle Heller‘s half-excellent coming-of-age tale, “The Diary Of A Teenage Girl.” Provocative, brutally honest, R-rated formative year stories for females are certainly in short supply, and so Heller’s vividly drawn debut feature certainly delivers in this regard, with a rich and expressively effervescent bildungsroman story. But like so many Sundance narratives this year, Heller’s movie begins to overstate its case and loses hold of its charms in its darker, overlong second half, yet manages some deft navigation of potentially distasteful subjects and tricky source material.

Based on cartoonist Phoebe Gloeckner‘sgraphic novels, “The Diary Of A Teenage Girl” is about a sexually precocious adolescent girl in 1970s San Francisco who begins a complex affair with her mother’s boyfriend. The film undoubtedly introduces us to some great new talent: Minnie Goetze (an outstanding Bel Powley) is a typical teenage girl. She’s curious, wants to be loved, and is trying to discover who she is. But the artistic and inquisitive Minnie is perhaps a little bit more sexually curious than most girls her age, and the anything-goes culture around her is certainly not disapproving of this exploration. Her carefree mother Charlotte (Kristen Wiig) parties and uses drugs liberally with her boyfriend Monroe (Alexander Skarsgård).

Already enamored with “the handsomest man in the world” Monroe, Minnie’s new normal sets the stage for a fateful night when the two cross the line. They quickly launch into a complicated, torrid love affair where both Minnie and Monroe have to tiptoe around their feelings. Excited about losing her virginity and dying to tell someone about the exhilarating clandestine world of adulthood she’s entered, the wide-eyed Minnie begins to document all her thoughts about love, sex, and Monroe through her art, perhaps dictating far too much evidence into her tape recorder. As you might imagine, it doesn’t end well.

Heller has technique to burn, employing inventive elements of animation mixed with live-action as Minnie uses her burgeoning sexuality and womanhood to find her place. Sharply observed, funny, and textured, the opening acts of “Diary Of A Teenage Girl” are endearing, clever, and intoxicating, and Minnie is completely believable.

As Monroe, the laid back but exploitative dude who begins to unexpectedly catch feelings, Alexander Skarsgård is quite good. Wiig doesn’t have as much to do, but after many frustratingly uneven serious performances of late (“The Skeleton Twins,” “Hateship Loveship“), she nails this role. A particularly hilarious scene-stealer is Christopher Meloni, as Pascal, Minnie’s mom’s uptight psychologist ex-boyfriend who continues to insist upon being a father figure to Minnie and her nosy sister Gretel (Abby Wait). But ‘Teenage Girl’ is for the most part the Bel Powley show. This British actress is amazingly genuine, and the movie rests on the shoulders of her effortlessly charming performance. She anchors the movie and it wouldn’t work half as well as it does without her.

‘Teenage Girl’ features good aesthetics encapsulating the counterculture era, with a cool soundtrack (The Stooges, T-Rex, Heart, Nico, Mott The Hoople) and great art direction, despite the period-accurate browny, mustard color palate. But at 102-minutes, “The Diary Of A Teenage Girl” simply can’t sustain its vivacious nature. Heller’s picture overdoes its “I just want to be loved” theme, and the romantic obsessiveness of the second half becomes frustrating. We want to see Minnie’s exploration of adulthood, and not always how it simply relates to Monroe.

As Minnie’s life begins to fall apart, she starts to use sex as a self-destructive weapon instead of a tool for self-discovery. While that seems fair enough given this particular crisis, the movie begins to take on miserablist qualities common to Sundance, and what began as idiosyncratic and fresh starts to feel more familiar.

Still, Heller is due a lot of credit. She takes objectionable, potentially repulsive subject matter and imbues it with honesty, fairness, and compassion without prejudice. Heller’s film can also be heartbreakingly authentic in its depiction of teenage wonder, infatuation, confusion, and insecurity. Powley’s poised, incredibly convincing portrayal also connects very real adolescent ideas of desire and exploration to the emotional inability to manage the fallout. Perhaps a little editing could have fixed some of the nagging second half issues, and it might have fulfilled its promise of being great and not just very good. Nevertheless, as uneven as it can be at times in its last 15 minutes, Marielle Heller has crafted a promising debut that evokes the idea of unlocking the secret world of teenage girls and letting us live inside the special little jewel box, if ever so briefly. [B]